The present invention relates to planetary gear systems, and more specifically to planetary gear systems used in automatic transmissions of motor vehicles.
Japanese Patent Provisional (unexamined, KOKAI) Publication No. 52-149562 discloses one conventional example. A planetary gear system of this example uses three single pinion type planetary gear sets as shown in FIG. 14. The planetary gear system has six rotatable members M1, M2, M4, M5, M6 and M7. The third member is named M4 instead of M3 in order to facilitate the comparison with the present invention. The member M7 is a driving member including an input shaft, and connected to the members M1, M4, respectively, through first and second clutches C1 and C2. The members M6, M5 and M4 are connected to the stationary housing through first, second and third brakes B1, B2 and B3, respectively. The member M2 is a driven member including an output shaft.
FIG. 15 is a nomographic chart of the gear system shown in FIG. 14. In this chart, the ratio of the horizontal distance between the positions of the members M2 and M4 to the horizontal distance between the positions of the members M1 and M2 is equal to A, and the ratio of the horizontal distance between the positions of the members M4 and M5 to the horizontal distance between the positions of the members M1 and M2 is equal to B. The ratio of the horizontal distance between the positions of the members M5 and M6 to the horizontal distance between the positions of the members M7 and M5 is equal to C. The ratios A, B and C are determined according to sizes a.sub.1, a.sub.2 and a.sub.3 of the first, second and third planetary gear sets. The size a.sub.1 (=the number of teeth of the sun gear/the number of teeth of the ring gear) of the first planetary gear set constituted by the members M5, M6 and M7 is equal to C. The size a.sub.2 of the second planetary gear set constituted by the members M1, M4 nd M5 is B divided by (1+A). The size a.sub.3 of the third planetary gear set constituted by the members M1, M2 and M4 is equal to A. Thus: EQU a.sub.1 =C, a.sub.2 =B/(1+A), a.sub.3 =A
In this conventional planetary gear design, however, it is very difficult to obtain satisfactory gear ratios. The first, second, third, fifth and sixth forward gear ratios and the reverse gear ratio are related by three constraint conditions regarding a.sub.1, a.sub.2, a.sub.3, and determined by the three variables A, B and C. Therefore, it is very difficult to make all the gear ratios equal to desirable values simultaneously. When, for example, the first speed gear ratio is set equal to 3.0, the second gear ratio is set equal to 1.9, and the third gear ratio is set equal to 1.32, then A=0.5, B=0.61, and C=0.954, and consequently, the fifth speed gear ratio becomes equal to 0.704, and the reverse gear ratio becomes equal to 2.50. This value of the fifth gear ratio is outside the desired range between 0.76 and 0.82, and the value of the reverse gear ratio is also outside the desired range between 2.25 and 4.2. Thus, the resulting fifth gear ratio is so low (high gear), that the driving power becomes insufficient at high vehicle speeds. The gear ratios of the different speeds are related to one another. Therefore, it is not possible to improve the fifth gear ratio without adversely affecting some other gear ratios. For example, it is possible to make the fifth gear ratio equal to a desired value of 0.760, by setting C equal to 1.6. In this case, however, the third gear ratio becomes equal to 1.223, and the reverse gear ratio becomes 1.983. These results are not in the respective desired ranges.